1. Field of the Invention
The present invention relates to an image pickup device which picks up an image of an object by use of an XY address type solid-state image pickup element, and a signal processing method for the image pickup device.
2. Description of the Related Art
In a solid-state image pickup element 1 of an XY address type represented by a CMOS (complementary metal oxide semiconductor) sensor, a large number of pixels are arrayed in a matrix of lines and columns, as shown in FIG. 1. The element 1 has a vertical scanning circuit 54 to sequentially select lines of a pixel part 52, a horizontal scanning circuit 60 to sequentially select columns of the pixel part 52, and an output circuit 61 to output signals.
The vertical scanning circuit 54 and the horizontal scanning circuit 60 are constituted by, for example, shift-registers, and generate one vertical scanning pulse and one horizontal scanning pulse for every line and column, respectively.
When reading image signals charged in respective pixels, a pulse signal is added to one vertical selection line 53 by the vertical scanning circuit 54, to electrically conduct all pixel transistors 51 in one line. Image signals are read from respective photosensitive parts 50 onto vertical signal lines 55. The image signals read onto the vertical signal lines 55 are fed to circuits such as a correlative double sampling circuit (CDS) 56 and the like to remove an offset signal for every pixel.
The horizontal scanning circuit 60 adds a pulse signal to transistors 57 connected to the respective vertical signal lines 55 through horizontal selection lines 59, to electrically conduct the transistors 57. Pixel signals from a column from which offset signals have been removed by the CDS 56 are read onto a horizontal signal line 58. The signals are converted into voltage signals by an output circuit 61, and are outputted to the outside.
In an XY address type solid-state image pickup element of this kind, the columns respectively have different (or their own) vertical signal lines 55. Hence, pixel signals fed from the output circuit 61 are offset differently between individual columns if the CDSs 56 and the transistors 57 have characteristic variants. Various offsets which are respectively specific to the columns appear, as stripe-like fixed-pattern noise (hereinafter called column noise) on a display screen, and cause deterioration in image quality.
There has been a method for preventing this deterioration. In the method, only the column noise component is extracted from the solid-state image pickup element, and is maintained as a reference signal for correction. During a normal image pickup operation, the reference signal is subtracted from a signal output from the solid-state image pickup element, to correct column noise.
However, when the pixel part 52 is irradiated with light, a signal component based on the incident light is added to the column noise component. Therefore, this output signal cannot be used as a reference signal for correction. Hence, according to Japanese Patent Application Laid-Open Publication No. 10-126697, the pixel part 52 is constituted by an effective pixel area A, a vertical optical black (hereinafter abbreviated as VOPB) area B, and a horizontal optical black (HOPB) area C, as shown in FIG. 2. The effective pixel area A is irradiated with light. In the vertical optical black area B and the horizontal optical black area C, irradiation of light is shielded over several lines to several ten lines by a light-shield plate such as an aluminum thin film, conducting detection and correction of column noise.
Meanwhile, the offset of each column which causes column noise appears in outputs not only from pixels positioned in the effective pixel area A but also from pixels positioned in the VOPB area B as a shielded black level reference. Therefore, correction can be achieved in a manner that column noise components are detected by use of signals from the pixels in the VOPB area B and are subtracted from signals from the pixels from the effective pixel area A.